1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, for example, a band-pass filter or a resonator, and the present invention also relates to a method of producing such a surface acoustic wave device.
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been widely used as band-pass filters and resonators. For application of surface acoustic wave devices as band-pass filters, resonators, or other such devices, the most important requirement is that the devices must have good frequency characteristics.
Generally, appropriate materials (electrode materials) for constructing the interdigital transducers (“IDTs”) and reflectors of the surface acoustic wave devices are different from each other, depending on the surface acoustic wave mode to be used and the particular application for the device. For example, as appropriate materials for the IDTs and reflectors of resonators and band-pass filters using Shear Horizontal or SH waves, metals such as Au, W, Ta, and Pt, having a specific gravity of 15 or higher are used in many cases.
Moreover, in the resonators, the band-pass filters, and such devices, in order to suppress vibration modes that deteriorate the characteristics thereof, the thicknesses of the IDT electrodes and reflectors must be within a predetermined range in some cases. If the film thicknesses of the IDT electrodes and the reflectors have predetermined values so that the vibration modes that deteriorate the characteristics can be initially suppressed, the film thicknesses will significantly depart from the values at which the surface acoustic waves can be most effectively vibrated and reflected. Thus, in some cases, a problem arises in that the necessary good characteristic cannot be obtained.
Moreover, if the film-thicknesses of the IDT electrodes and the reflectors are restricted, it will be more difficult to sufficiently reduce the resistances of the IDT electrodes, and also, problematically, the devices cannot be sufficiently adapted for operation at a high frequency.
Referring to production of multiple filters and resonators using the same wafer, it is desirable that the frequency dispersion be made as small as possible. For this purpose, advantageously, materials having a specific gravity of up to 10, such as Ni, Cu, Al, Mg, and Co, are used. However, when these materials are used, it is difficult to obtain filters and resonators having desired characteristics in practice.
Moreover, when filters and resonators including IDT electrodes and reflectors using metals with a specific gravity of 15 or higher, such as Au, W, Ta, and Pt, are produced, efforts are made to form the IDT electrode and reflector films as uniformly as possible so that the frequency dispersion of multiple surface acoustic wave devices (filters, resonators, and other such devices) produced by using the same wafer can be suppressed. However, there is a limit to the reduction in thickness of the IDT electrodes and the reflectors. Practically, the wafer is divided in individual elements, and thereafter, frequency-adjustment is carried out for each of the elements. The frequency-adjustment is made because a slight difference in film thickness between the electrode films constituting the IDT electrodes and the reflectors causes the acoustic velocity to change significantly, thereby increasing the frequency dispersion of the resonators and filters.
In order to achieve the frequency-adjustment, a method of etching the surface of an IDT electrode by using ion beams, a method of forming an insulator film on a substrate and an IDT electrode, a method of etching a substrate or an IDT electrode by RIE, are generally used. Accordingly, a problem arises in that the manufacturing cost is increased due to reduction of throughput, and moreover, the characteristics deteriorate due to damage to the electrodes and the substrates.